[Bug testsuite/104423] [libgomp, testsuite] Add means to do accelerator-only testing in libgomp

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104423

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Thomas Schwinge from comment #2)
> For OpenMP test cases, we'd either have to manually mark them up (error
> prone and generally ugly), or scan the source file(s) (error prone and
> generally ugly), or use some kind of marker that gets set during compilation
> if any offloading code has been generated.

I was thinking along the lines of already existing markers in the exec:
...
$ nm for-3.exe | grep offload
                 U GOMP_offload_register_ver
                 U GOMP_offload_unregister_ver
0000000000a59718 D __offload_funcs_end
0000000000a58a58 d .offload_func_table
0000000000a58a58 D __offload_func_table
0000000000a5ca38 D __offload_vars_end
0000000000a59718 d .offload_var_table
0000000000a59718 D __offload_var_table
...

Say, if the exec doesn't have a __offload_func_table, it can be skipped.

This would allow to do one test run, where you run all the tests, and generate
the offloading tests list, and then several more runs (using the same sources,
so the list is still up to date) where you just do the accelerator tests.